A touch switch having a capacitive touch sensor as an operation switch for various devices is well known. A touch operation with respect to the touch switch (i.e., a contact operation of a human body such as a finger of a person) is detected as follows. Specifically, a capacitance under a condition that the human body does not contact an electrode of the touch sensor is defined as a standard value (i.e., a base line level). It is determined based on a result (i.e., a difference) for comparing a detection value of the capacitance of the touch sensor with the base line level whether the human body touches or not.
The capacitive touch sensor may cause unwanted operation, i.e., operation error according to the influence of an external electric wave noise. To prevent the operation error, an immunity measure is implemented such that a determination standard is optimized by updating the base line level periodically, so that the influence of the noise is removed as much as possible. However, it may be difficult for this measure to prevent the operation error with respect to an instantaneous noise.
Further, the patent literature No. 1 discloses a technique, as an immunity measure, for comparing the maximum change of the capacitance and the minimum change of the capacitance among multiple switches (i.e., touch sensors), and detecting and controlling a touch operation with using an absolute value of a difference between the maximum change and the minimum change of the capacitance.
The method described in the patent literature No. 1 may be considered as an effective immunity measure if a switch using a touch sensor has a layout such that the switch is arranged in a comparatively narrow area. However, in the method described in the patent literature No. 1, a possibility for failing to prevent the operation error caused by the noise may be high if a localized noise is generated when the switch using the touch sensor has a layout such that the switch is arranged in a comparatively wide area.